The power consumption of a flash memory device has become an important issue as the portable electronic device has become increasingly popular in recent years. A conventional flash memory device has a memory array with a great number of memory units (i.e. memory cells). Each of the memory units is formed as a field-effect transistor including a floating gate and a control gate. The floating gate is configured to hold charges, and is fabricated on an oxide layer over an active region of a semiconductor substrate. The floating gate is separated from the source/drain regions in the semiconductor substrate by the oxide layer. Each memory cell may be programmed or electrically charged by injecting electrons into the floating gate from the substrate through the oxide layer. The charges can be removed from the floating gate during an erase operation. The data in each of the memory cells may be judged by the charges accumulated in the floating gate. Important quality indexes of the flash memory include the power consumption and reliability thereof.